Suppression of tunneling two-level systems in ultrastable glasses of indomethacin

Glasses are disordered solids usually obtained by supercooling a liquid bypassing crystallization. A remarkable feature of glasses is that, independently of their nature and composition, they exhibit universal properties in the low-temperature range. Of interest here, the specific heat is characterized by a linear term below 1 K, ascribed to quantum tunneling between two states of similar energy. We have investigated if this ubiquitous behavior also applies to so-called “ultrastable glasses,” directly synthesized from the vapor phase into low-energy positions of the potential-energy landscape. Interestingly, we find a full suppression of the linear term in the specific heat, which questions the current view of the popular tunneling model and sheds light on the microscopic origin of two-level systems in glasses.